This invention relates to superabsorbing polymers based on L-aspartic acid in synthetic fiber and film forms and to process(es) for preparing such fibers and films.
Polysuccinimide (PSI) is prepared by thermal polycondensation of L-aspartic acid which can then be base-hydrolyzed to polyaspartate salt which has many industrial uses such as lubricant in metalworking fluids. Crosslinking PSI before or after hydrolysis renders the hydrolyzed salt superabsorbent in that it can absorb many times its weight of liquid such as water. This capability of absorbing significant quantities of fluids, including body exudates and aqueous compositions of all kinds, creates another important class of application for these polymers in products such as diapers, sanitary napkins, incontinence products, towels, tissues and the like. These superabsorbing polymers are in the prior art as typically disclosed in U.S. Pat. No. 5,461,085 (Nagatomo et al); U.S. Pat. No. 5,525,703 (Kalota) and U.S. Pat. No. 5,612,384 (Ross et al). Though articles of superabsorbing polymers derived from L-aspartic acid are recognized in this art as desirable, to date they are only disclosed in unshaped, particulate form as recovered from the hydrolysis step forming the salt. Note all the examples of the patents referenced above. Continued absence in the art of these polymers in fiber or film form is likely due to the important conflicting requirements of being sufficiently plastically extensible to permit drawing fiber or forming film while being crosslinked and hydrolyzed to create superabsorbency.
In view of the noted applications, it would be highly desirable to provide biodegradable superabsorbent polymers derived from L-aspartic acid in fiber or film form to facilitate formation into products such as diapers and the like.
The superabsorbent fibers and films of the present invention provide a solution to many problems encountered with unshaped, particulate superabsorbent polymers derived from L-aspartic acid. For example, the superabsorbent fibers of the invention have the advantages of (1) avoiding the problem of migration by having the ability to entangle with fluff pulp fibers, (2) being compatible with other fibers resulting in greater flexibility in applications and fabrication, and (3) having a large surface area resulting in a faster absorption rate. For example, the superabsorbent films of the invention have the advantage of avoiding the problem of migration by having the ability to remain stationary within an article containing the film.
Now, significant developments have been made in producing superabsorbent polymer fibers and films derived from L-aspartic acid.
Accordingly, a principal object of this invention is to produce synthetic, superabsorbent fibers and films of L-aspartic acid derivatives.
Another object is to provide a method for producing such fibers or films.
A further object is to chemically modify PSI to render it capable of drawing into fiber or forming film.
Yet another object is to provide an intermediate precursor for production of fibers or films based on PSI prepared from L-aspartic acid.
Other objects will in part be obvious and will in part appear from the following detailed description.
These and other objects are accomplished by the following multiple aspects of the invention.
i) A fiber- or film-forming plastic composition must be sufficiently plastically extensible to permit forming into film or drawing into a filament-like shape without fracture. It is difficult to draw fibers or form films from crosslinked and hydrolyzed PSI which has not been further modified. This problem is remedied by one aspect of the invention by forming an uncrosslinked, non-hydrolyzed, internally plasticized poly(imide-co-amide) intermediate precursor which is useful in later forming such superabsorbing polymeric fibers or films. The intermediate is prepared by reacting a regulated amount of about 1 to about 20% of succinimide groups of the PSI with a minor, internally plasticizing amount of one or more monoamine compounds, such as diethanolamine. The partial amidation with the monoamine compound softens the polymer and facilitates subsequent drawing into fiber form or forming into film. For example, the poly(imide-co-amide) intermediate produced using diethanolamine as the monoamine compound has repeating structural units represented by formula (1) 
and repeating structural units represented by formula (2) 
the mole fraction of repeating structural units represented by the formula (1) being about 0.01 to about 0.20.
ii) The internally plasticized poly(imide-co-amide) intermediate is subsequently hydrolyzed to convert essentially all of the succinimide groups into aspartate groups. A portion of the aspartate groups in the polyamide produced as a result of the hydrolysis are then partially acidified to produce a partially acidified, hydrolyzed, internally plasticized polysuccinimide composition.
iii) Since fiber- or film-forming polymer cannot be effectively drawn or formed when crosslinked, another aspect of the invention incorporates crosslinking agent into the partially acidified, hydrolyzed, internally plasticized polysuccinimide composition before shaping fiber or film from the crosslinkable but uncrosslinked intermediate composition. Crosslinking aspartate groups of the polymer occurs after fiber or film formation in an after-treating or curing step by subjecting the fibers or film to crosslinking conditions to render them capable of superabsorbing. Premature crosslinking is minimized or avoided by incorporating heat reactive crosslinking agents into the composition at low temperature, i.e. from or about 0xc2x0 C. to or about 25xc2x0 C.
iv) Products of the process differ from those previously known in that the superabsorbing polymer is importantly in fiber or film form. The crosslinked fibers or film are of polyamide containing at least three divalent or polyvalent moieties randomly distributed along the polymer chain having the following formulas: 
where A represents hydrogen, alkali metal cation, ammonium, quaternary ammonium or mixtures thereof, R represents a divalent or polyvalent crosslinker moiety, x, y and z represent mole fractions of the moieties in the polyamide and are respectively about 0.01 to 0.20; about 0.40 to 0.90 and about 0.01 to 0.50 wherein the sum of x, y and z is 1.0, and n is an integer from 0 to 4. R1 and R2 are substituents on the monoamine compound used for the internal plasticization of PSI and can be the same or different. Optionally, the superabsorbent polymer contains minor amounts of unreacted succinimide repeating units, i.e. repeating unit disclosed in formula (2) above, and unreacted acidified aspartate repeating units, i.e. repeating unit disclosed in formula (4) herein. As used herein, a minor amount of succinimide repeating units or acidified aspartate repeating units is an amount up to that amount which has a detrimental effect on the absorbency properties of the superabsorbent fiber or film.
v) In a specific aspect of the invention, a process is provided for preparing superabsorbing fibers or film which comprises the following steps in the recited sequence:
i) condensation polymerizing L-aspartic acid to form polysuccinimide (PSI) having a weight average molecular weight of at least about 20,000 Daltons;
ii) reacting about 1 to about 20% of the succinimide groups of the PSI with one or more monoamine compounds to form an internally plasticized poly(imide-co-amide) intermediate;
iii) hydrolyzing essentially all of the succinimide groups of the poly(imide-co-amide) to form an internally plasticized polyamide;
iv) acidifying about 1 to 50% of the aspartate groups of the polyamide of iii);
v) admixing crosslinker with the partially acidified polyamide of iv) under non-crosslinking conditions to form a crosslinkable, uncrosslinked, partially acidified, hydrolyzed, internally plasticized PSI composition;
vi) drawing fibers or forming film from the composition of v); and
vii) subjecting the fibers or film to crosslinking conditions to crosslink uncrosslinked aspartate groups and form the superabsorbing fibers or film.
Synthetic superabsorbing polymer fibers and films of the invention are derived from L-aspartic acid starting monomer available commercially from Solutia Inc. L-aspartic acid is conventionally condensation polymerized in the presence of catalyst such as phosphoric acid. Processes for preparing homopolymer polysuccinimide (PSI) are described in U.S. Pat. Nos. 5,057,597; 5,315,010 and 5,319,145. Molecular weight (weight average Mw) is preferably at least 20,000 and more preferably at least 30,000 up to or about 100,000 Daltons. Such relatively high molecular weight is achieved by driving the polycondensation reaction to as complete a level as commercially feasible using catalyst concentrations, reaction temperature and time at the high end of the ranges disclosed in these patents. Water of condensation is removed as it is formed as taught in U.S. Pat. No. 5,484,945 (Nagatomo et al) the disclosure in which is incorporated herein by reference. In a preferred procedure, polycondensation is conducted at reduced pressure and 180xc2x0 C. in the presence of 85% phosphoric acid as described in U.S. Pat. No. 5,142,062 (Knebel et al) the disclosure of which is also incorporated herein by reference. Succinimide (xe2x80x9cSxe2x80x9d) groups of formula (2) are the repeating structural unit.
To permit formation of fibers or films from the composition (as will be later described), S groups of the PSI are next ring-opening reacted with one or more monoamine compounds having the formula HNR1R2 in an amount functionally effective to internally plasticize the PSI and form a poly(imide-co-amide) intermediate. R1 represents a hydrogen atom or an alkyl or alkenyl group of 1 to 55 carbon atoms, preferably 1-30, which can be straight chain or branched and unsubstituted or substituted, and R2 represents a hydrogen atom, xe2x80x94OH, an alkyl or alkenyl group of 1 to 55 carbon atoms, preferably 1-30, which can be straight chain or branched and unsubstituted or substituted. The alkyl or alkenyl groups of R1 and R2 optionally contain one or more oxygen atoms. Optional substituents of the alkyl or alkenyl groups of R1 and R2 are common organic functional groups not interfering with the hydrolysis, acidification or crosslinking reactions of the invention such as one of the following: hydroxyl (xe2x80x94OH), ether (xe2x80x94OR3), chloride (xe2x80x94Cl) and ketone (xe2x80x94COR3), wherein R3 represents an alkyl or alkenyl group of 1 to 8 carbon atoms. Currently preferred substituents of the invention are xe2x80x94OH and xe2x80x94OR3.
The amount of monoamine compound in the poly(imide-co-amide) is that amount necessary to achieve adequate internal plasticization of the PSI and varies with the specific monoamine chosen. The amount of monoamine necessary is readily determined by one of ordinary skill and is based on the properties of the specific monoamine, e.g. molecular weight. Using diethanolamine as the monoamine, the repeating structural unit is represented by formula (1) in combination with S groups of formula (2), the mole fraction of repeating structural units represented by formula (1) being about 0.01 to about 0.20. The internal plasticizer softens and provides the polymer with fiber- and film-forming properties. Any compound containing one functional amino group which is reactable with PSI can be used to form the poly(imide-co-amide) intermediate and provide the internal plasticizing function. Compounds with two or more reactable amino groups tend to lead to crosslinking and therefore should be avoided. Other monoamino compounds interchangeably usable with the diethanolamine of the Example following illustratively include, but are not limited to, methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, Oxe2x80x94(2-Aminopropyl)xe2x80x94Oxe2x80x2xe2x80x94(2-methoxyethyl)polypropylene glycol 500 (Jeffamine(copyright) M-600), ethanolamine, neopentanolamine, 3-ISONONYLOXYPROPYLAMINE, 3-propanolamine, 2-methoxy-ethylamine, 3-methoxy-propylamine, 3-ethoxypropyl-amine, ethylhexoxy-propylamine, isopropanolamine, and diisopropanol-amine. Monoamine is reacted with PSI in a solvent mixture at a temperature adequate for succinimide ring-opening which is typically about 40 to 70xc2x0 C. Suitable solvents for the succinimide ring-opening reaction are water, polar organic solvents such as dimethylformamide (DMF), dimethylsulfoxide, and N-methyl-2-pyrrolidone (NMP), and non-polar organic solvents such as toluene and hexane. The preferred solvents for the succinimide ring-opening reaction are water and polar organic solvents, with water being the most preferred solvent. Using diethanolamine the reaction is illustrated as follows: 
wherein a and b represent the mole fractions of the respective repeating structural units, and a is 0.01 to 0.20 and b is 0.99 to 0.80.
Internally plasticized PSI or poly(imide-co-amide) is next hydrolyzed with a regulated amount of base sufficient to form salt from essentially all of the S groups of the poly(imide-co-amide) to form an internally plasticized polyamide. As used herein, the term xe2x80x9cessentially allxe2x80x9d means greater than about 99%. The repeating unit of hydrolyzed succinimide, i.e. aspartate, has the following structure: 
where N is an alkali metal cation such as Na+, K+, Li+, ammonium or quaternary ammonium. This hydrolysis is accomplished by reacting the poly(imide-co-amide) reaction product of the prior process step with a suitable base, e.g. alkali metal hydroxide, ammonium hydroxide, and the like, in a suitable solvent selected from water, polar organic solvents such as DMF, DMSO and NMP, non-polar organic solvents such as toluene and hexane, and mixtures thereof. The currently preferred solvent is water and the currently preferred base is sodium hydroxide. In a preferred embodiment, this hydrolysis is conveniently accomplished by adding aqueous base solution in situ to the poly(imide-co-amide) reaction product of the prior process step. In the preferred embodiment, the hydrolyzed, internally plasticized PSI composition is totally dissolved in water solution after completion of the hydrolysis. Hydrolysis occurs at room temperature or, to reduce reaction time at elevated hydrolysis temperature typically up to about 75xc2x0 C., until essentially all of the S groups are hydrolyzed.
The hydrolyzed, internally plasticized PSI composition, i.e. the internally plasticized polyamide, is next partially acidified with a regulated amount of an acid sufficient to convert about 1 to 50% of the aspartate groups into the acid form, i.e. acidified aspartate groups, for use in the crosslinking reaction. The amount of acid to partially acidify the hydrolyzed, internally plasticized polyamide is that amount necessary to reduce the pH to less than about 6.5, preferably from about 4 to about 6. The repeating unit of acidified aspartate has the following structure: 
This partial acidification is accomplished by reacting the internally plasticized polyamide reaction product of the prior process step with a suitable acid in a suitable solvent selected from water, polar organic solvents such as DMF, DMSO and NMP, non-polar organic solvents such as toluene and hexane, and mixtures thereof. Suitable acids are acids that are capable of achieving a pH of less than 5 in the partial acidification reaction mixture and include mineral acids, e.g. hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid and the like, and organic acids, e.g. carboxylic acids. The currently preferred solvent is water and the currently preferred acid is hydrochloric acid.
Crosslinker for eventually crosslinking acidified or non-acidified aspartate groups is then admixed under non-crosslinking conditions into the solution of partially acidified, hydrolyzed, internally plasticized PSI to form a crosslinkable, uncrosslinked, partially acidified, hydrolyzed, internally plasticized, PSI composition. The crosslinker can be admixed with the solution of partially acidified, hydrolyzed, internally plasticized PSI before or after concentration of the solution. In the preferred embodiment, crosslinker is admixed under non-crosslinking conditions into an aqueous solution of partially acidified, hydrolyzed, internally plasticized PSI to form a crosslinkable, uncrosslinked, partially acidified, hydrolyzed, internally plasticized PSI composition. According to the invention, crosslinking is delayed until after fiber or film formation, but crosslinker is added before fiber or film formation to insure that the crosslinker is evenly distributed throughout the partially acidified, hydrolyzed, internally plasticized PSI solution. Adding crosslinker to the solution while minimizing or avoiding crosslinking is accomplished by doing so at or about room temperature (22-25xc2x0 C.) down to or about 0xc2x0 C. This relatively low temperature protects against premature crosslinking before fiber or film formation and can vary with the crosslinking activity, or reactivity, of the crosslinker. Such non-crosslinking temperature conditions are chosen to avoid significant development of gel which occurs when crosslinked polyaspartate salt absorbs solvent, e.g. water, from the solution. Such gel should be avoided since fibers cannot effectively be drawn nor films effectively formed from a gel-containing partially acidified, hydrolyzed, internally plasticized PSI composition.
Suitable crosslinkers for the partially acidified, hydrolyzed, internally plasticized PSI composition according to the invention are any suitable polyfunctional compound having two or more functional groups that will react with the carboxylate groups of at least two of the aspartate groups at the pH conditions of the crosslinking reaction. Suitable crosslinkers include, but are not limited to, polyepoxides, haloepoxides (particularly chloroepoxides such as epichlorohydrin), polyaziridines, polyoxazolines, and mixtures thereof. As used herein, polyepoxides include compounds having two or more epoxide groups, e.g. diepoxides, triepoxides, and tetraepoxides. As used herein, haloepoxides include compounds having two or more functional groups wherein at least one functional group is an epoxide group and at least one functional group is a halogen. As used herein, polyaziridines include compounds having two or more aziridine groups and polyoxazolines include compounds having two or more oxazoline groups. Suitable polyepoxide crosslinkers of the invention include, but are not limited to, those represented by the formula: 
wherein xe2x80x9cexe2x80x9d is 2 to 6, and R10 is selected from a linear or branched aliphatic radical having 2 to 30 carbon atoms, an alicyclic radical having 3 to 18 carbon atoms, or an aromatic radical having 6 to 26 carbon, wherein the radicals optionally contain one or more oxygen atoms. The R10 radical will have a valency equal to xe2x80x9cexe2x80x9d. As used herein, the term xe2x80x9caromaticxe2x80x9d includes, but is not limited to, groups such as phenyl, naphthyl, pyridyl and the like in which the ring may be substituted by groups which do not interfere with the crosslinking reaction such as, but not limited to, C1 to C6 alkyl, nitro, halo, C1 to C12 alkoxy and the like. As used herein, the aliphatic and alicyclic groups are optionally substituted by groups which do not interfere with the crosslinking reaction such as, but not limited to, nitro, halo, hydroxy, C1 to C12 alkoxy and the like. When xe2x80x9cexe2x80x9d is 2, the linear or branched aliphatic radicals preferably have 2 to 14 carbon atoms and the alicyclic radicals preferably have 3 to 12 carbon atoms. When xe2x80x9cexe2x80x9d is 3, the linear or branched aliphatic radicals preferably have 3 to 18 carbon atoms and the alicyclic radicals preferably have 4 to 12 carbon atoms. When xe2x80x9cexe2x80x9d is 4, the linear or branched aliphatic radicals preferably have 5 to 30 carbon atoms and the alicyclic radicals preferably have 5 to 18 carbon atoms. When xe2x80x9cexe2x80x9d is 5, the linear or branched aliphatic radicals preferably have 6 to 30 carbon atoms and the alicyclic radicals preferably have 6 to 18 carbon atoms. When xe2x80x9cexe2x80x9d is 6, the linear or branched aliphatic radicals preferably have 8 to 30 carbon atoms and the alicyclic radicals preferably have 8 to 18 carbon atoms.
Examples of polyepoxides for use in the invention include, but are not limited to, ethylene glycol diglycidyl ether, 1,4-cyclohexane dimethanol diglycidyl ether, diglycidyl 1,2-cyclohexane dicarboxylate, N,N-diglycidyl-4-glycidyloxyaniline, and 4,4xe2x80x2-methylenebis(N,N-diglycidylaniline).
The polyepoxides of the invention are readily available or can be prepared by processes known in the art, such as by epoxidation of polyolefin with peracid.
Suitable polyaziridine crosslinkers of the invention include, but are not limited to, those represented by the formula: 
wherein R4 is an alkyl group having 1 to 10 carbon atoms which is optionally substituted by groups which do not interfere with the crosslinking reaction such as, but not limited to, nitro, halo, hydroxy, C1 to C12 alkoxy and the like; R5 is an aliphatic radical having 1 to 30 carbon atoms or a direct bond; X is an alkylene group having 1 to 30 carbon atoms, optionally containing an ester group, an ether group, an amide group or a similar inert group; and xe2x80x9caxe2x80x9d is 2 to 4. Preferred polyaziridines are those in which R4 is methyl, ethyl, propyl or butyl, X is represented by the formula 
wherein b is 1 to 3 and c is 1 to 3, a is 2 to 3, and R5 is a propylene radical.
Examples of polyaziridines for use in the invention include, but are not limited to, trimethylolpropane tris[(xcex2-N-aziridinyl)propionate], and pentaerythritol tris[(xcex2-N-aziridinyl)propionate].
The polyaziridines of the invention can be prepared by processes known in the art such as by dehydration of xcex1-amino hydroxyl compounds.
Suitable polyoxazoline crosslinkers of the invention include, but are not limited to, those represented by the formula: 
wherein R7 and R8, which may be the same or different, represent hydrogen, an alkyl radical having 1 to 8 carbon atoms or an aryl radical having 6 to 12 carbon atoms; R9 represents a polyfunctional, more particularly difunctional, alkylene radical having 1 to 20 carbon atoms, preferably 1 to 12 carbon atoms, or an arylene radical having 6 to 12 carbon atoms; and d is 1 to 3.
Examples of polyoxazolines for use in the invention include, but are not limited to, ethylenebis(2-oxazoline), 1,2,4-tris(2-oxazoline) butane, and 2,2xe2x80x2-methylenebis[(4S)-4-phenyl-2-oxazoline].
The polyoxazolines of the invention are prepared by processes known in the art.
The crosslinker is preferably used in anhydrous (neat), undiluted, virgin form as a solid or liquid, but alternatively can be a component of a dilute or concentrated solution, dispersion or suspension. A currently preferred crosslinker is ethyleneglycol diglycidyl ether. The amount of crosslinker according to the invention is that amount which is sufficient to crosslink a portion of the acidified or non-acidified aspartate groups of the polyamide precursor corresponding to about 1 to about 30%, preferably about 1 to about 15%, of the S groups in the initial homopolymer PSI. The preferred amount of crosslinker will depend on the specific crosslinker used. The acidified or non-acidified aspartate groups crosslinked during curing comprise from about 1 to about 30%, preferably about 1 to about 15%, of the total succinimide groups originally present in the PSI. This amount and the resulting eventual level of crosslinking renders the polymer of the fibers or film superabsorbing in being capable of absorbing from at least 3 times to more than 100 times their weight of water. Though not wishing to be bound to any particular structure, it is believed the crosslinking agent exists as an unreacted component in the uncrosslinked composition which may be intimately admixed with the balance of the components of the composition, or at most is chemically reacted via one functional group of the crosslinker to one carboxylate group of an aspartate group but not to two which would create a crosslink undesirably leading to gel formation.
At this stage a partially acidified, hydrolyzed, internally plasticized, crosslinkable, uncrosslinked PSI composition exists for formation into fibers or film in a manner about to be described which comprises:
i) repeating, internally plasticized structural units represented by formula (3) 
ii) repeating acidified aspartate structural units represented by formula (4) 
iii) repeating aspartate structural units represented by formula (5) 
and
iv) crosslinking agent, as described herein, capable under crosslinking reaction conditions of crosslinking units of formula (4), wherein M represents alkali metal, ammonium, quaternary ammonium or mixtures thereof, R1 and R2 are as defined above, and x, w and y represent the mole fractions of structural units (3), (4) and (5) and are respectively about 0.01 to 0.20; about 0.50 to 0.01 and about 0.40 to 0.90 wherein the sum of x, w and y is 1.0. Optionally, the partially acidifed, hydrolyzed, internally plasticized, crosslinkable, uncrosslinked PSI contains minor amounts of unreacted succinimide repeating units.
For the preparation of fibers, the uncrosslinked solution, which is preferably an aqueous solution, is thickened to about 50% polymer solids concentration and a thin film of this concentrated mixture (solution) manually applied with a spatula or pipette at room (about 22xc2x0 C.) temperature to the surface of a 2.5xc3x9715 cm metal plate. A second plate of equal dimension is pressed against the film on the first plate and as the two plates with the interposed film of concentrated mixture are manually moved apart at a slow rate at room temperature, the film is drawn into single, long, thin, filament-like shapes which are initially joined to each plate but then fracture as the plates move further apart to form elongated, shaped fibers. The unreacted crosslinker present on the polymer fibers is substantially homogeneously distributed. The fibers (which are essentially non-superabsorbing at this stage) are subjected to crosslinking conditions of elevated temperature and time sufficient to cure and crosslink uncrosslinked aspartate groups of the polymer and provide the fibers with superabsorbing capability. An alternative fiber-forming system employing a spinning die to continuously form and then cure fibers according to this invention is described in U.S. Pat. No. 4,855,179 (Bourland et al), the fiber-forming and curing disclosure of which is incorporated herein by reference.
For the preparation of film, the uncrosslinked solution, which is preferably an aqueous solution, can be formed into a film by any conventional film-forming process. For example, the uncrosslinked solution can be thickened, e.g. to about 5% to about 50% polymer solids concentration, preferably about 10% to about 30%, extruded into a gas atmosphere while evaporating the solvent to form the film, the film stretched, and then the film so formed crosslinked. The concentration of the polymer in the uncrosslinked solution is selected, having regard to the molecular weight of the polymer, so as to give a solution having a viscosity that is convenient for extrusion through the extrusion die being used.
In another embodiment, the solution can be deposited upon a support to form a wet film of the solution. The nature of the support is not critical and may be selected from a variety of materials depending on the particular application including, but not limited to, polymeric (e.g. in extruded, film and porous matrix forms), ceramic, glass, or metallic supports. The preferred supports are the polymeric supports, particularly those in porous matrix form. Numerous techniques are available for the application of the solution to the support as will be apparent to those skilled in the art. For example, the polymer solution may be simply poured upon a level support in a quantity sufficient for it to achieve the desired uniform thickness. A blade may then be drawn over the surface of the wet film to aid the deposition of the wet film of uniform thickness. The thickness of the wet film deposited upon the support is determined by the desired thickness of the film ultimately produced. Generally, the wet film is deposited upon the support in a substantially uniform thickness of about 2 to about 30 mils, preferably about 4 to about 10 mils. A quantity of solvent is next evaporated from the exposed surface of the wet film to allow the formation of a relatively thin solid layer of the exposed surface of the film. During the formation of the solid layer of the exposed surface of the film, the solvent present near the surface of the wet film is flashed off and a thin coagulated skin of polymer remains. The evaporation of solvent from the exposed surface of the wet film may be accomplished by a variety of techniques as will be readily apparent to those skilled in the art. For example, a stream of air or other gas at ambient or an elevated temperature below the point at which the polymer in the film will crosslink may be simply directed at the exposed surface of the wet film. The time required to form the desired thin solid layer upon the exposed surface of the wet film commonly ranges from about 30 minutes to about 5 hours, preferably about 30 minutes to about 1 hour. The film is then crosslinked as described herein.
Products of the curing step are partially acidified, hydrolyzed, internally plasticized, crosslinked, superabsorbing fibers or film derived from polysuccinimide. The crosslinked fibers or film are formed of polyamide containing at least three divalent or polyvalent moieties randomly distributed along the polymer chain of the following formulas: 
where A represents hydrogen, alkali metal cation, ammonium, quaternary ammonium or mixtures thereof, R represents a divalent or polyvalent crosslinker moiety derived from the crosslinker used, x, y and z represent mole fractions of the moieties in the polyimide and are respectively about 0.01 to 0.20; about 0.40 to 0.90 and about 0.01 to 0.50 wherein the sum of x, y and z is 1.0, and n is an integer from 0 to 4. R1 and R2 are substituents on the monoamine compound used for the internal plasticization of PSI and can be the same or different. Optionally, the superabsorbent polymer contains minor amounts of unreacted succinimide repeating units, i.e. repeating unit disclosed in formula (2) above, and unreacted acidified aspartate repeating units, i.e. repeating unit disclosed in formula (4) above.
The superabsorbent fibers and film of the invention are useful in the manufacture of moisture absorbent articles, such as disposable diapers, sanitary napkins, incontinence garments, bandages, absorbent liners in meat packing trays, pet tray liners, and the like. The superabsorbent fibers and film of the invention are particularly useful in the manufacture of thin or ultra-thin disposable diapers which have excellent moisture absorbance capacity, fluid distribution properties and reduced leakage. The superabsorbent fibers of the invention are also useful directly or in non-woven sheet or matting form for agricultural or gardening materials such as water-holding materials for soils, e.g. mixing the fibers directly with soil. The superabsorbent films of the invention are also useful directly or attached to a porous matrix for agricultural or gardening materials such as water-holding materials for soils. The sheet or matting form can also be used for seedlings and landscaping applications.
In making absorbent articles with the superabsorbent fibers of the invention, the fibers may be mixed with, attached to, layered in, or dispersed in a porous matrix of fibers. In one embodiment, the superabsorbent fibers of the invention are combined with other fibers to form a nonwoven material. The superabsorbent fibers of the invention can be combined with hydrophilic fibers such as cellulose pulp or fluff, cotton liners, and synthetic fibers or a mixture of the fibers and the cellulose fluff. The fibers can be loose or joined as in nonwovens. Suitable synthetic fibers include, but are not limited to, polyesters, copolymers of polyesters and polyamides, polyvinyl alcohol and the like. The synthetic fibers may be meltblown fibers or fibers which have been treated to render them hydrophilic. Additionally, the superabsorbent fibers of the invention may be incorporated in the absorbent article in a compartment or localized area in the absorbent structure.
In making absorbent articles with the superabsorbent films of the invention, the films may be attached to or layered in a porous matrix of fibers or a porous film. The superabsorbent films of the invention can be combined with hydrophilic fiber matrices comprising cellulose pulp or fluff, cotton liners, and synthetic fibers or a mixture of the fibers and the cellulose fluff. Suitable synthetic fibers include, but are not limited to, polyesters, copolymers of polyesters and polyamides, polyvinyl alcohol and the like. The synthetic fibers may be meltblown fibers or fibers which have been treated to render them hydrophilic. Additionally, the superabsorbent films of the invention may be incorporated in the absorbent article in a compartment or localized area in the absorbent structure.
Absorbent articles for use in hygienic and sanitary products, such as disposable diapers, are made with a liquid-impermeable backing material, a liquid-permeable bodyside facing material and the liquid-absorbing material sandwiched between the backing material and the facing material. The liquid-impermeable backing material can be made from commercially available polyolefin film and the liquid-permeable facing material can be made from a commercially available nonwoven material, such as spunbonded or corded fibrous web which is wettable and capable of passing the fluid to be absorbed, e.g. urine.